Recent advances in the analysis of joint seakeeping processes are applied to the accumulation of spectral fatigue cycles. The methods of cross co-spectral moments and techniques adopted from the determination of principal angles for seakeeping processes are used to determine orientation of the most severe plane for the accumulation of spectral fatigue cycles and the distribution of stress cycles on that plane. The theory presented considers all aspects of the wave loads experienced over the life of the structure and uses linear theory to predict lifetime fatigue using more efficient techniques than equally comprehensive time-domain simulation. Three methods of determining the orientation of the principal plane and their theoretical differences are discussed. Quantitative comparisons are presented in the time, frequency and probability domains to illustrate differences and demonstrate consistency.